Mix the concrete as the instructions says on the package. Remove the middle part of the matchstick box, the cardboard part (if your box has such a part). Then pour the concrete into the matchstick box. Take the paper clip and break it apart. Widen the ends of the clip so that you get it at an angle. Put the pieces of paper clip in the concrete, a little obliquely, and then wait until the concrete is dry. Done!